Engines, an example of an internal combustion engine, have been reduced in size, and there are growing needs for an increased low-speed torque and improved responsiveness. A multi-stage centrifugal compressor is attracting attention as an approach to meet such needs (see Patent Document 1). A multi-stage centrifugal compressor has a rotational shaft extending from either side of a rotary driving unit, a low-pressure stage compressor disposed on one end of the rotational shaft, and a high-pressure stage compressor connected to the opposite end of the rotational shaft and configured to re-compress intake air compressed by the low-pressure stage compressor.
If an electric motor is employed as the rotary driving unit of the above multi-stage centrifugal compressor, when the electric motor operates to drive the low-pressure stage compressor and the high-pressure stage compressor, intake air compressed by the low-pressure stage compressor has its temperature increased and generates heat, and so does intake air compressed by the high-pressure stage compressor. Accordingly, heat is accumulated in the multi-stage centrifugal compressor, and the electric motor may break down.
Thus, a motor housing that retains an electric motor is normally equipped with a plurality of heat-dissipating plates. Further, a centrifugal compressor utilizing a centrifugal force can be easily reduced in size, and thus an operation control part that controls operation of an electric motor is sometimes provided accommodated in a centrifugal compressor.